The present invention relates to a variable load brake control device for rail vehicles and particularly for rail vehicles having a brake control system based on the pressure equalization principle of brake control.
Specialized freight vehicles, such as convertible rail/road cars that are adaptable for hauling by a locomotive in rail service and a truck tractor in highway service, typically employ air springs between the car body and underframe for optimal cushioning in both rail and road service. Having air springs that reflect the car load in accordance with the air spring pressure, these specialized cars lend themselves to variable load brake control as opposed to the more traditional empty/load type brake control employed in freight type rail service. The advantage of variable load type brake control over empty/load type control is that regardless of the vehicle load weight, a braking ratio can be achieved whereby the maximum braking effort approaches, but does not exceed the wheel/rail adhesion threshold, irrespective of the vehicle load condition.
These specialized freight cars are equipped with conventional direct release type freight brake control valves, such as the well-known ABD type control valve, for controlling the vehicle brakes when operating in rail service. The direct release ABD type control valve operates on the pressure equalization principle, in which a slide valve is positioned by the service valve piston, in response to a brake pipe pressure reduction effective on one side, to connect compressed air from an auxiliary reservoir to the car brake cylinder(s). When this auxiliary reservoir pressure effective on the opposite side of the service valve piston decreases slightly below the brake pipe pressure, the service piston moves to lap position in which a slide valve interrupts further flow of compressed air from the auxiliary reservoir to the brake cylinder(s). The auxiliary reservoir and brake cylinder volumes are chosen so that, when a predetermined full service reduction of brake pipe pressure is made, complete equalization will occur therebetween and no further increase in brake cylinder pressure can be obtained short of making an emergency brake application. A control valve of the aforementioned type offers the advantage of lower cost than alternative type proportional control valves.